The present invention relates to a metallic bellows and in particular to a method of making a metallic bellows.
Metallic bellows have a wide range of applications, and a typical application is found in pressure accumulators owing to its capability to seal off gas and withstand repeated extension and contraction. Conventionally, a metallic bellows was typically made by welding together appropriately shaped metallic sheets. However, this fabrication method is not suited for mass production because of the difficulty in carrying out the welding process with a required precision and uniformity. For this reason, stamp formed bellows have come to be preferred over the more conventional welded bellows. A bellows can be stamp formed most conveniently by introducing pressurized liquid into an enclosed metallic tube blank which is surrounded by a suitable metallic die assembly, and this may be called as the hydraulic bulge method.
The stress of the bellows can be computed from the well-known formula specified in xe2x80x9cJapan Industrial Standards (JIS) B 8243 Structure of Pressure Vesselsxe2x80x9d and given in the following.
"sgr"x=1.5xc3x97txc3x97xcex94xxc3x97E/{(xcfx81/2)0.5xc3x97h1.5xc3x972n}xe2x80x83xe2x80x83(1)
where "sgr"x: the stress produced by the extension and contraction of the bellows (MPa), t: plate thickness (mm), xcex94x: axial deflection (mm), E: Young""s modulus (179 GPa in the case of SUS 304), n: effective number of annular bulges, xcfx81: pitch (mm), and h: height of each bulge (mm).
As can be appreciated from this formula, increasing the height of each annular bulge is effective in reducing the stress of the bellows. The bulge height can be given by (outer diameterxe2x88x92inner diameter)/2, and the inner diameter is given by the inner diameter of the metallic tube blank. Therefore, by increasing the outer diameter/inner diameter ratio, the bulge height can be increased and the stress of the bellows can be reduced. Also, for the given permissible stress, by increasing the height of each annular bulge, the number of bulges can be reduced, and the axial length of the bellows can be thereby reduced. This contributes to a compact design, and enables the bellows to be used in a limited space. For instance, a pressure accumulator using such a bellows can be made highly compact, and the freedom in the accumulator design can be enhanced.
However, according to the prior art, the height of each annular bulge was limited by the capability of the material to elongate. In other words, if an attempt is made to achieve a bulge height which is more than the maximum elongation of the material permits, the material ruptures. Therefore, conventionally, the bulge height was only so large as the elongation of the material permitted, and could not be increased so much as desired.
For instance, when SUS304 is used for making a bellows by the conventional hydraulic bulge method, due to the limit in the elongation of the material, the ratio of the outer diameter to the inner diameter (D1/D2) cannot be any more than about 1.5. This puts a limit to the possible stroke of the bellows for the given size of the bellows.
Such a problem can be mitigated by using materials capable of larger elongations. However, a material demonstrating a larger elongation is relatively expensive, and this increases the manufacturing cost. Alternatively, instead of using a forming process, the bellows may be fabricated by the welding method which provides a greater freedom in design without being encumbered by such a limitation. However, when making a bellows by welding, it is necessary to weld the circumference of each of a plurality of annular thin plates, and this complicates the manufacturing process. This not only increases the manufacturing cost but also causes some difficulty in ensuring the required capability to withstand repeated loads due to the unavoidable variations in the quality of welding.
In view of such problems of the prior art, a primary object of the present invention is to provide a method of making a bellows which provides a larger stroke for a given size.
A second object of the present invention is to provide a method of making a bellows which provides a larger stroke for a given level of stress.
A third object of the present invention is to provide a method of making a bellows which provides a larger stroke for a given selection of material.
According to the present invention, such objects can be accomplished by providing a method of fabricating a bellows by bulge forming, comprising the steps of: placing a metallic tube blank in a first die assembly; introducing pressurized fluid into the metallic tube blank so as to form a plurality of annular bulges in the metallic tube blank in cooperation with the first die assembly; removing the metallic tube blank from the first die assembly and annealing the metallic tube blank; placing the annealed metallic tube blank in a second die assembly; and introducing pressurized fluid into the metallic tube blank so as to further bulge out the annular bulges of the metallic tube blank in cooperation with the second die assembly.
According to this method, by conducting the annealing step during the bulge forming process of the bellows, the capability of the material to elongate is recovered, and the workability of the bellows is improved in effect so that an additional forming step can be conducted upon the bellows which has been subjected to the previous forming step.
In particular, the ratio of the outer diameter to the inner diameter can be made greater than a value that can be achieved by a single forming step. Therefore, the deflection (stroke) of each annular bulge or pleat can be increased for a given stress or, in other words, the number of annular bulges can be decreased and the length of the bellows can be decreased for a given stroke of the bellows.
Typically, the metallic tube blank is made of stainless steel, but other materials can be used without departing the spirit of the present invention. Also, the first and second die assemblies may consist of a common die assembly, instead of being two different die assemblies.
According to a preferred embodiment of the present invention, the die assembly comprises an upper die component, a lower die component and a plurality of intermediate annular die components arranged between the upper and lower die components at an equal interval. In particular, each of the intermediate annular die components is preferably provided with an annular ridge defining annular recesses on either side thereof, the recesses of the intermediate annular die components jointly defining an outer profile of the annular bulges of the metallic tube blank. Preferably, the intermediate annular die components are adapted to be brought closer to each other uniformly as the pressurized fluid is introduced into the metallic tube blank. Each of the upper and die component is preferably provided with a plug that fits into a corresponding axial end of the metallic tube blank in a liquid tight manner. This allows the interior of the metallic tube blank to be conveniently sealed off, and the liquid for pressurization can be introduced in to the interior of the metallic tube blank from a passage formed in one of the plugs.